The present invention relates to modifying the surface of the inner walls of pores in objects such as membranes to be used for separating material or phases of matter, preferably for filtering biological liquids or liquids containing organic molecules under further utilization of an activator which traverses the pores. Membrane materials in relation to which the invention can be practiced include for example polysulfone, polycarbonate, polypropylene, polyethylene, polyamid, polyurethane, cellulose, senorized hydrocarbons such as polytetrafluoroethylene (see Pusch and Walch in Angew. Chem. 94 (1982)670-695).
Membranes made of synthetic material or of regenerated natural material are used for example for filtering biological liquids or other liquids containing organic molecules in general. Solutions in question are for example protein solutions such as whey, waste liquid which arises in the process of paper making, sugar refining or gelatin manufacture; still other liquids are oil emulsions, etc. In all these instances, the problem arises that through the adsorption of organic molecules such as proteins, on the surface of the membrane, i.e., the inner walls of pores therein, the cross-section of flow is in effect reduced so that the throughput or throughflow of matter through the membrane is diminished and the efficiency of operation is reduced accordingly.
Known membranes exhibit a further problem, namely the curious property that most synthetic membranes are hydrophobic. Therefore, during filtering of watery solutions, one needs a certain minimum pressure to overcome the capillary depression in the pores in order to be able to force water into and through the pores at all. The hydrophobic phenomenon as stated provides a quite undesired capillary depression which is of disadvantage in the process of watery solution. On the other hand, such a capillary depression may in other instances be quite desirable. For example, in the manufacture of textiles for making for example, waterproof clothing, canvas or the like, the problem arises that the material should be water-tight, but should still be permeable to air and water vapor in order to avoid impediment of transpiration. Textiles having these properties have in common with the membranes of the type mentioned above that generally they are to provide for a separation of substances and/or phases of matter. Therefore, it is quite reasonable to presuppose that technologies which are suitable and impart upon membranes these particular desired properties and conversely avoid the above-mentioned undesirable properties, can also be used for textile processing.
It is known that membranes may require particular chemical and/or physical properties. It is also known to impart upon the membranes these properties by surface modification, generally understood. For example, undesired absorption can be reduced, for example, through coating with carbon or through grafting of particular functional groups, such as sulfonate groups. The wettability of hydrophobic membranes will be improved, i.e., the requisite capillary depresssion is reduced and the requisite operating pressure is reduced in that prior to filtration a tensid is placed on the membrane. However, a hydrophilization thus achieved is only temporary in nature because the tensides adsorb only physically and do not bind chemically to the surface of the membrane. A permanent hydrophilization, however, is attainable through chemically grafted polar groups on the surface of the membrane.
Vacuum or wet-coating are other methods being used for surface treatment. However, vacuum methods are disadvantaged by the fact that they do not permit coating of the inner walls of pores. Wet-coating is usable in some instances, but because of solution and swelling problems, they are not suitable for all kinds of membrane. Moreover, a controlled coating of the inner walls of pores is made difficult by variations in the local concentration of the particular modifying reactants of the solution so that indeed the coating provided on the inner walls of the pores is very inhomogenous. In view of diffusion it is hardly possible to maintain a definite concentration of the reactants at boundaries. Moreover, solution media which tend to swell the membrane material were also observed to transpose the reaction from the surface into the interior, i.e., the surface near regions of the pore walls of the membrane.
Finally, it is known to modify a membrane surface with the use of a plasma, again in order to provide the membrane with desirable hydrophilic properties. These kinds of modification methods are indeed effective only on the outer surfaces of a membrane, but not in the interior surfaces of the pores. On the other hand, the pores, and particularly their inner walls materially influence the behavior and characteristics of the membrane, particularly as far as hydrophilic hydrophobic properties are concerned. Therefore, it is necessary to extend any kind of modification method into the interior pore surfaces of a membrane. In order to obtain this result, it is known to treat the membranes with chemically or biochemically active liquid substances which are forced through the pores. (See for example, German printed patent applications Nos. 2615815 and 2650921). These methods are of course wet-coating methods. However, the utilization is limited whenever the aforementioned disadvantages of the wet-coating method generally are encountered.